Unsaturated monomers of the N-vinylamide class have been employed in free radical polymerization reactions for the preparation of homopolymers and copolymers having a range of useful properties. Of these, the N-vinyllactams, in particular N-vinyl-2-pyrrolidone, have been employed extensively owing to their wide commercial availability and rapid polymerization with a range of comonomers including the acrylic and methacrylic esters. Polymerizations of other cyclic and acyclic N-alkyl substituted (tertiary) N-vinyl carboxylic acid amides have been described as well. Secondary N-vinylamides such as N-vinylformamide and N-vinylacetamide also copolymerize well with acrylic compounds and are useful for preparing hydrophilic polymers which may be subsequently hydrolyzed to introduce vinylamine functionality.
Several N-vinylamide monomers have been suggested as components of liquid and solid photopolymerizable compositions of various types. As a class, the vinylamides possess certain useful properties for these applications, including good compatibility with other chemistries, copolymerizability with acrylates, comparative resistance to oxygen inhibition, and favorable adhesion-promoting characteristics.
Lorenz et al. U.S. Pat. No. 4,129,709 disclose a coating composition comprising N-vinyl-2-pyrrolidone, an acrylated oligomer, and an acrylic acid ester having a boiling point of at least 200.degree. C. at 760 mm Hg. These compositions may be cured by exposure to actinic radiation between 200 and 750 nm or by an electron beam. Tu et al. U.S. Pat. No. 4,319,811 describe radiation curable coatings consisting of triacrylate or tetraacrylate monomers with an N-vinyl imido monomer, preferably an N-vinyllactam such as N-vinyl-2-pyrrolidone. Priola and coworkers U.S. Pat. No. 4,348,427 describe compositions comprising mixtures of acrylated oligomers and/or unsaturated polyester oligomers with a least one unsaturated compound of the amide, lactam, piperidone and urea classes, and curing them by exposure to ultraviolet radiation in the 200-400 nm range.
Cornforth et al. U.S. Pat. No. 5,281,682 teach improved radiation-curable formulations containing N-vinylformamide and an oligomer selected from the group epoxy acrylates, urethane acrylates, polyester acrylates and mixtures thereof. Elzer et al. U.S. Pat. No. 4,725,524 disclose a dry film photoresist containing an acrylic or methacrylic oligomer, a compatible film-forming water-soluble polymer, one or more compatible photopolymerizable monomers, a photoinitiator, and other additives. Barzynski et al. U.S. Pat. Nos. 4,205,139 and 4,424,314 teach curable compositions containing N-vinyl compounds in which at least two N-vinyl groups are present and in which at least one carbonyl group is bound to the nitrogen of the N-vinyl group, said carbonyl group in turn being bonded to a nitrogen or carbon atom.
The most commercially important class of radiation-curable compositions relies upon the free radical photopolymerization of unsaturated acrylic compounds. These systems are commonly based on acrylic ester-terminated oligomers derived from one of several major resin chemistries, such as urethanes, epoxies, polyesters, and others. The acrylated oligomers are often compounded with various nonpolymerizable materials (pigments, fillers, flow agents, antioxidants, etc), as well as photoinitiators and co-catalysts, and applied to a substrate before curing. Curing is accomplished by exposing the formulation to ultraviolet light or other type of radiation until a dry adherent polymerized film is formed. Formulations of this general description find use as printing inks, protective coatings, adhesives and the like.
In practice, it is often necessary to incorporate diluent monomers into these formulations in order to lower the viscosity of the oligomers sufficiently to permit adequate flow and leveling on the substrate prior to irradiation. This is particularly true of formulations applied by methods common to the printing and coating industries. Diluent monomers used for this purpose fall into two broad catagories: multifunctional (or polyunsaturated) types and monofunctional (or monounsaturated) compounds. Multifunctional monomers generally provide high cure speed and high crosslink density, leading to hard, chemically-resistant films, but they may not sufficiently lower viscosity and may also contribute to poor adhesion due to excessive shrinkage of the films on curing. Monofunctional monomers are usually more effective in reducing viscosity, and can yield softer, more extensible films that exhibit better adhesion to many substrates.
Although a large number of unsaturated compounds are potentially useful as diluents, relatively few have proven to be practical. This is because reactive monomers employed in this technology must conform to a number of physical and performance requirements, among them: high fluidity, low vapor pressure, low color, low odor, high flash point, low toxicity and irritancy, broad compatibility and rapid copolymedzability with the other unsaturated species.
Monounsaturated N-vinylamides have the aforementioned useful properties as photocurable compounds, however, few of these monomers meet all the criteria for use as diluents. It is found, for example, that the lower alkyl tertiary N-vinylamides (e.g. N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide) are excellent solvents, but are excessively volatile and possess relatively low flash points (under 200.degree. F.). The higher secondary and tertiary N-vinylamides (e.g. N-vinylacetamide, N-vinylbenzamide, N-vinylcaprolactam, N-phenyl-N-vinylacetamide, N-vinyl-2-piperidinone, N-vinylsuccinimide, N-vinylmorpholinone) are less volatile and less hydrophilic, but have melting points above room temperature, making them less desirable as diluents or coreactants in liquid systems. Many of these monomers exhibit unacceptably slow cure rates as well.
Kurtz and Disselnkoetter Liebigs Ann. Chem., 764, p. 69-93, (1972) describe the preparation of 3-(N-vinylformamido)propionitrile by the reaction of N-vinylformamide with acrylonitrile in benzene utilizing a potassium cyanide catalyst. Applications for the compound are not reported. However, this material has also been found to have certain deficiencies as a photocurable monomer. A further disadvantage is the use of a highly toxic catalyst in its preparation.
Of the N-vinyl class, only N-vinyl-2-pyrrolidone (NVP) has achieved widespread commercial acceptance in the technology owing to its satisfactory combination of good solvent properties, low vapor pressure and high cure rate in liquid photocure systems. More recently, N-vinylformamide (NVF) has been shown to offer equal or superior performance. However, both NVP and NVF have certain disadvantages as well. Notably, both monomers possess relatively high glass transition temperatures (150.degree.-175.degree. C.) which imposes some contraints upon the film properties that can be achieved. In addition, both monomers are relatively hydrophilic which can undesirably increase the water sensitivity of cured films.
It is the object of this invention to provide novel N-vinyl monomers which simultaneously exhibit the desirable solvent and polymerization characteristics of the N-vinylamides, and which also possess the required physical properties necessary to render them practical as components of liquid radiation-curable formulations, and which further possess a wider range of Tg and hydrophobic properties than is exhibited by the preferred liquid monomers of the prior art, i.e. NVP and NVF.